考量產能限制之供應鏈層級存貨監督機制

Abstract

由於企業競爭日益激烈，企業與其他企業的整合越趨重要，但是每條供應鏈中各成員所擁有的權力大小並不相同，且目標常以自身利潤極大化為主，而不以整條供應鏈之效益極大化為目標，造成供應鏈的績效不佳，且難以進行監控，另外，產能利用率和其他績效因素有交互關係，因此過高或過低的產能利用率對企業皆有不良影響，因此，在考量產能限制下，在以滿足顧客服務水準為目標下，供應鏈中須共享資訊，進行有效整合，提升顧客滿意度以及整體供應鏈效率。 本研究應用層級存貨(Echelon inventory)觀點以及CONWIP系統概念於供應鏈，以建構一供應鏈監控機制。本研究先以達到服務水準目標為前提，考量生產特性和CONWIP控制對在製存貨量的影響，並且探討產能利用率、生產週期以及在製存貨量之間關係，再分別以前端和後端成員為主之角度，建立多層級存貨監控模式，建構最小化供應鏈二層級在製存貨模型；再以遞迴求解以及提高內部層級的服務水準之概念發展之演算法，並在以後端為主之模式中，以二元搜尋法找尋適當的內部服務水準，將二層級的模式擴展至多層級，得到多層級在製存貨上界。且利用求解模型所得到之各層級在製存貨上界作為監控指標，經由模擬驗證本研究所設定的多層級在製存貨監督上界之有效性。經由模擬驗證的結果有下列結論： (1)藉由本研究提出以遞迴求解發展之演算法，可有效得到多層級的在製存貨上界，以後端為主之供應鏈在製存貨上界略大於以階段觀點得到之上界，以前端為主之供應鏈，則小於以階段為主所得之上界； (2)相較於過去研究，本研究考量產能之限制下，以層級在製存貨取代階段在製存貨作為監督上界，使整體供應鏈系統能在不增加過多在製存貨量的情況下，得到較佳的服務水準。主要的原因為系統中在製存貨配置之不同，以及利用整體的資訊來進行規劃所致。

With fiercer and fiercer competition, the integration among the enterprises is getting more and more important. The power which the members owned in the supply chain is not equal and the goals of the members of supply chain always pursue for their own profits, not the profits of whole supply chain. As a result, this leads to poor performance of supply chain. Besides, capacity utilization effects on some other performance indexes with each other. Too high or low capacity utilization makes negative effects. Therefore, considering capacity limitation, in order to enhance the service levels and improve the performance of the whole supply chain, it is essential for all members to share their information and synchronize their operations. This research attempts to develop a supply chain monitoring scheme based on the concepts of echelon WIP inventory and CONWIP system. The first step in the proposed scheme is to construct a two-echelon model which takes the features of the production system into consideration with the goal of minimizing the echelon WIP inventory control limits of the supply chain under the service level constraints. Next, the model is extended to a multi-echelon model through raising the service levels of the internal echelons and solving the model iteratively. The algorithm searches the proper internal service level with the concept of binary search. With the resulting echelon WIP inventory control limits from the model, the proposed monitoring scheme has been validated through simulation study. Based on the simulation validation results, the conclusions are drawn as follows: (1) By raising the service levels of internal echelons, the proposed scheme can effectively derive the multi-echelon WIP inventory limits, which is lower than the stage-based WIP inventory limits of the supply chain which is predominated by the back-end members but higher than the front-end members; (2) Compared to the traditional stage-based inventory monitoring scheme, the proposed echelon-based monitoring scheme can obtain a higher service level under the lower inventory levels. This is the result of reallocating of the WIP limits and the utilizing of overall information.